1. Field of the Invention
This invention deals with the removal of metals such as lead, cadmium, mercury, arsenic, zinc, chromium, tin iron and cobalt from compositions containing such metals as organic and inorganic compounds. The need for the removal of such metals is evidenced by the fact that their presence in hydrocarbon charge stocks conducted to catalytic cracking and catalytic reforming units is known to poison and shorten the life of the catalyst with which such metal contaminated stocks come into contact.
It is also desirable to remove trace metals from lubricating oils or to recover soluble metal catalysts from reactor effluents of polymer solutions. The removal of heavy metals such as mercury, silver, cadmium and the like from the water effluents of chemical, electrolysis, metal deposition or photographic plants is also highly desired from an ecological standpoint.
It is known that residues of alkyl lead moieties from combustion of leaded gasoline tend to poison catalysts available for cleaning automotive exhaust gases by the catalytic oxidation of carbon monoxide and unburned hydrocarbons in the exhaust. Such poisoning severely shortens the useful life of exhaust combustion catalysts. It has thus been heretofore proposed that substantially metal-free, and primarily substantially lead-free gasoline be supplied for use in automobiles equipped with emission control devices which utilize catalysts to help further oxidize exhaust gases.
The normal network of petroleum product distribution involves railroad tank cars, pipelines, water borne tankers, tank trucks and bulk storage tanks. For commercial operation these are presently set up to handle different products. For example, the same pipeline might be used to convey a shipment of regular grade gasoline, premium grade gasoline, distillate fuel and other light liquid products in succession. According to present procedures, that portion of the fluids carried by the pipeline which constitutes an intermingling of the two products at their interface will be diverted to storage for the lower grade product, thus avoiding degradation of the higher grade product.
However, when leaded and/or when metal containing gasoline is followed by metal-free gasoline, not just the interface comprising an intermingling of the two products, but the entire lead-free shipment becomes degraded. When leaded gasoline, containing tetraethyl lead, tetramethyl lead or a mixture or transalkylation product of the two is contacted with the metal surfaces of transportation and storage facilities, a significant amount of lead is left deposited in scale and on the metallic surfaces. Upon later using the same facilities for lead-free gasoline the latter product becomes contaminated to an extent which may run as high as about 0.07 gram of lead per gallon. This amount of lead is sufficient to impair the life of exhaust emission control oxidation catalysts.
2. Description of the Prior Art
Techniques have heretofore been known for removal of dissolved or suspended heavy metal contaminants from liquid products.
In catalytic cracking and reforming operations, the use of guard chambers containing a variety of sorbents intended to remove heavy metal contaminant from the charge stock before contact is made with the catalyst have been described.
Systems for removal of lead from gasoline have also been proposed. Presently known techniques require considerable time or are non-selective in effecting removal from the gasoline of those additives which are desired to be retained, such as anti-oxidants, anti-icing additives, metal passivators and the like.
One previously proposing system for removing lead is described in U.S. Pat. No. 2,386,261. There, acid activated clay such as bentonite which has been treated with hydrochloric or sulfuric acid is used. Leaded gasoline is percolated through the clay to remove 95% of the lead present. Acid activated clays will also remove the additives which are required for proper protection and functioning of automotive equipment.
Another approach is that described in U.S. Pat. No. 2,392,846. According to an example in this patent, a five gallon lot of leaded gasoline is treated with 20 ml. of stannic chloride followed by addition of 100 grams of activated carbon. This results in decomposition of the tetraalkyl lead and adsorption of the lead decomposition product on the activated carbon thus drastically reducing the lead content. The gasoline is removed from the activated carbon by decantation. This is a very slow process which permits the processing of about 35 gallons of gasoline per hour. Here also even the additives desired to be retained in the gasoline will also be adsorbed by the activated carbon.
Both the processes described in the cited prior patents depend for effectiveness on a chemical conversion of the tetraalkyl lead. The lead compounds can be reacted with such materials as halogens, halogen acids, metal halides, metal salts, sulfur dioxide, carboxylic acids, metals in the presence of hydrogen, etc. The resulting decomposition products are not readily soluble in hydrocarbons and hence are selectively adsorbed on high surface adsorbents. This avoids the property of tetraalkyl lead moieties which presents the greatest difficulty in this separation, namely infinite solubility in hydrocarbons.